Abstract
In this work, we report on the exciton radiative lifetimes of graphitic carbon nitride monolayers in the triazine- (gC$_3$N$_4$-t) and heptazine-based (gC$_3$N$_4$-h) forms, as obtained by means of ground- plus excited-state ab initio calculations. By analysing the exciton fine structure, we highlight the presence of dark states and show that the photo-generated electron-hole pairs in gC$_3$N$_4$-h are remarkably long-lived, with an effective radiative lifetime of 260 ns. This fosters the employment of gC$_3$N$_4$-h in photocatalysis and makes it attractive for the emerging field of exciton devices. Although very long intrinsic radiative lifetimes are an important prerequisite for several applications, pristine carbon nitride nanosheets show very low quantum photo-conversion efficiency, mainly due to the lack of an efficient e-h separation mechanism. We then focus on a vertical heterostructure made of gC$_3$N$_4$-t and gC$_3$N$_4$-h layers which shows a type-II band alignment and looks promising for achieving net charge separation.
Highlights
After the seminal work of Wang et al.[1] in 2009, proposing polymeric graphitic carbon nitride as a new, eco-friendly, low cost, and thermally stable photocatalyst for hydrogen evolution, this layered material has become the subject of intense research efforts[2−9] for possible employment in several applications[10−12] as a “green” replacement of more expensive, polluting, metal-containing compounds.[13−18] Among various allotropes of C3N4 with different densities,[19−21] the graphitic form has been proved to be the most stable at standard conditions.[22]
Several mechanisms to achieve this goal have been proposed, ranging from doping with metal and nonmetal atoms,[60,61] to order−disorder transitions,[62] to the creation of heterojunctions.[63−65] We show that a novel van der Waals (vdW) heterostructure, in which a gC3N4-t and a gC3N4-h layer are vertically stacked, results in an interface with a type-II band alignment, promising for obtaining net charge separation between the two layers
The phonon density of states (phDOS) computed for gC3N4-t and gC3N4-h in the flat 1 × 1 and 3 × 3 cells
Summary
After the seminal work of Wang et al.[1] in 2009, proposing polymeric graphitic carbon nitride as a new, eco-friendly, low cost, and thermally stable photocatalyst for hydrogen evolution, this layered material has become the subject of intense research efforts[2−9] for possible employment in several applications[10−12] as a “green” replacement of more expensive, polluting, metal-containing compounds.[13−18] Among various allotropes of C3N4 with different densities,[19−21] the graphitic form has been proved to be the most stable at standard conditions.[22] It is composed of two-dimensional (2D) layers of carbon and nitrogen atoms covalently bonded, stacked by means of van der Waals (vdW) interactions as in graphite These weak dispersion forces allow to exfoliate graphitic carbon nitride[23,24] into few-layers nanosheets or even singlelayer, graphene-like carbon nitride (gC3N4). While this behavior has been already discussed in the literature, as we will point out in the Received: December 14, 2020 Accepted: January 15, 2021 Published: January 27, 2021
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